I dunno, I used to do Republic a lot when I payed on Regent and below, now I commonly play on Monarch/Emperor, and my army is just too large to be supported by a Republic. I am a die-hard warmonger so that might have something to do with it. War weariness is another reason I prefer Monarchy. I will generally go Monarchy ASAP and stay there until I can go into Communism (as I play exclusively on Large or Huge maps my empire is generally very big by the industrial age). It's great fun to whip newly-captured cities down to about size 5 or so I have never played with feudalism because it struck me as a compromise between Monarchy and Republic and it seems generally better to go with one or the other.